Demand for digital subscriber line (DSL) service across existing twisted pair copper wires between a central office and a remote location is increasing. Typically, DSL services operate in accordance with DSL standards recommended by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU). A family of DSL Recommendations from the ITU includes: G.992.1, G.992.2, G.991.1, G.996.1, G.994.1, G.997.1 and G.995.1. Recommendation G.995.1 provides an overview of these standards. Recommendations G.991.1, G.992.1, G.992.2 have developed techniques for transmitting a range of bit rates over the copper wires of the local network including high bit rates at relatively short distances, and lower bit rates at longer distances. In particular, the G.992.1 and G.992.2 recommendations are based on asymmetric digital subscriber line technology that has different data rates in each direction of transmission. The G.992.1 recommendation is referred to as G.dmt and uses a splitter to filter the voicegrade signals at the remote location. The G.992.2 recommendation is referred to as G.lite and does not use a splitter. Recommendations G.994.1, G.996.1 and G.997.1 support the G.992.1 and G.992.2 recommendations by providing common handshake, management and testing procedures. These standards allow substantial flexibility in implementation.
DSL services typically use a discrete multi-tone (DMT) signal to transmit data. A DMT signal has multiple subchannels, each of which is assigned a frequency, also referred to as a carrier frequency or a tone, belonging to a discrete frequency band. Because individual subchannels operate at different frequencies, the subchannels may have different operating characteristics. For instance, more power may be used at higher frequencies. In addition, different numbers of bits may be loaded on different subchannels in accordance with their capacity, which depends on frequency, power, signal-to-noise ratio and transmission line characteristics. Subchannels that do not meet or exceed a minimum signal-to-noise ratio are not used. When initiating a DSL communication session, in the DSL modem, an initialization procedure at the receiver determines a number of bits to be grouped into a symbol for each subchannel, that is, a number of bits per subchannel, and exchanges that information with the transmitting DSL modem.
Quadrature amplitude modulation (QAM) is a technique to encode multiple bits into a QAM symbol. Each QAM symbol represents a distinct combination of bit values using a distinct combination of amplitude and phase of the carrier waveform based on distinct X and Y coordinates associated with respective bit values. Each QAM symbol is represented by a QAM waveform.
Referring to FIG. 1, a signal space diagram depicts an exemplary constellation 100 having a set of constellation points 102. Each constellation point 102 represents a distinct QAM symbol. The constellation points are mapped along an x-axis (X) 104 and a y-axis (Y) 106. Each constellation point 102 is associated with a distinct label which represents a respective distinct combination of bit values for a predetermined number of bits b. The label is also referred to as a tuple. Each constellation point 102 is also associated with a distinct (X, Y) coordinate. FIG. 1 depicts a constellation 100 having four constellation points 102 in which the number of bits b is equal to two. For example, the constellation point having the label 0 has an X coordinate equal to 1 and a Y coordinate equal to 1; and the constellation point having the label 3 has an X coordinate equal to −1 and a Y coordinate equal to −1.
In quadrature-amplitude modulation, the amplitudes of two quadrature carriers are modulated and the carriers are combined. The X coordinate represents the amplitude of a first carrier, and the Y coordinate represents the amplitude of a second carrier that is shifted in phase by 90° with respect to the first carrier. For example, the first carrier is a sine wave, while the second carrier is a cosine wave. Each constellation point 102 represents a distinct combination of the modulated carriers and thus a distinct QAM symbol.
A constellation encoder encodes groups of bits (tuples) into the (X, Y) coordinates of a QAM symbol. For example, tuples of four bits have sixteen possible distinct values. When encoding tuples of four bits, a constellation will have sixteen distinct constellation points, one constellation point for each value. The constellation encoder maps each of the sixteen possible combinations of values of the four bits to a distinct constellation point which has distinct (X,Y) coordinates which are used to produce the QAM symbols. The number of bits in the tuple, that is, the number of bits encoded, is referred to as the constellation size b.
In other words, for a given QAM channel, or in broadband communications for a given QAM subchannel, one typical constellation encoder selects an odd-integer point (X,Y) from a square constellation based on the values of the binary b-bit tuple, {vb-1, vb-2, . . . , v1, v0}. In the constellation, the b bits of the tuple are associated with an integer label having a binary representation of {vb-1, vb-2, . . . , v1, v0}. In FIG. 1, for example, when b=2, the four constellation points have labels of 0, 1, 2 and 3. Typically, the integer values of the X and Y coordinates of the constellation point are determined from the b bits such that the values of X and Y are odd integers having two's complement binary representations of { . . . , 1} and { . . . , 1), respectively. The most significant bit of X and of Y is the sign bit.
In digital multi-carrier systems, the digital information is transformed by a modem into an analog form that is essentially a sequence of DMT symbol waveforms. Each DMT symbol bears information in an array of zeroes and ones. The digital multi-carrier system is a channel having i subchannels. Each subchannel is associated with a distinct carrier frequency. The channel has i sub-arrays, one associated with each subchannel. Each sub-array has bi bits. Therefore, bi represents a number of bits per subchannel i. Each sub-array corresponds to a QAM waveform representing a constellation point in a 2b,-point constellation. A DMT symbol waveform is the superposition of these QAM waveforms. Each subchannel is characterized by a signal-to-noise ratio γi, where γi represents the signal-to-noise ratio (SNR) at the ith carrier frequency.
In DMT systems, each subchannel has a constellation encoder. Typically, in DMT systems, equal error protection is applied, and the number of bits per subchannel bi is determined such that, for each subchannel, the bit error rate should not exceed a target bit error rate prior to decoding and retransmission pb.
The number of bits per subchannel is selected while executing an initialization procedure. During initialization, the channel is analyzed and parameters are determined and exchanged. The ITU recommendations for G.lite and G.dmt set a target bit error rate (BER) standard of 10−7. The number of bits per subchannel are selected based on, at least in part, a target bit error rate standard and an average number of erroneous bits per QAM symbol.
In practice, the bit error rate for data transmitted using conventional G.992.1 and G.992.2 constellations may fluctuate. This fluctuation in the bit error rate reduces the number of bits per subchannel. Therefore, a method and apparatus are needed to reduce fluctuations in the bit error rate and thus increase the number of bits per subchannel.